Protective metal coating compositions and method of coating



United States Patent PROTECTIVE METAL COATING COMPOSITIONS AND METHOD OFCOATING Bruno R. Jeremias, Detroit, Mich., assignor to Poor & Company,Chicago, 11]., a corporation of Delaware No Drawing. Application July19, 19.56

Serial NO. 598,762 g 8 Claims. (Cl. 1486.21)

and improved corrosion protecting metal finishing com- 1 position whichproduces a beautiful bronze color on zinc surfaces, for example, diecast zinc articles.

Another object of the invention is to provide a composition of the typedescribed which produces enhanced corrosion protection on Zinc surfaces.

Another object of the invention is to provide a new and improved methodof producing excellent bronze corrosion resistant finishes on zincsurfaced articles. Other objects will appear hereinafter.

In accordance with the invention new and improved metal finishingcoating compositions are prepared by intimately mixing chromic acid, areducing agent capable of reducing a minor proportion of the chromicacid to a state in which the chromium is present as trivalent chromium,acetic acid, sulfuric acid and nitric acid. This composition isinitially prepared as a concentrate, preferably by mixing togetherchromic anhydride, sodium sul fite, acetic acid, sulfuric acid, nitricacid and water. The concentrate is then diluted in actual practice Witha relatively large quantity of water so that the final bath which isemployed as a finishing composition for metal surfaces only containsaround 1.5% to 4% by volume of the original concentrate.

The best mode contemplated for the practice of the invention isillustrated by the following composition which constitutes theconcentrate:

Ingredients: Grams per liter Chromic anhydride (CrO 401.0 Sodium sulfite(Na SO- 7.5 Acetic acid 22.5 Sulfuric acid 45.0 Nitric acid 228.0

the remainder being water.

When this composition is diluted with water so that the resultant bathcontains 1.5% to 4% by volume of the initial concentrate the said bathis an excellent corrosion protecting composition and provides abeautiful bronze finish on Zinc surfaced articles. The general procedurein providing such a finish on such articles is to immerse the articlesin the bath at ordinary temperatures (70 F. to 90 F.) for a period of 5to 15 seconds, then rinse the article with Water and dry.

In the practice of the invention the amount of chromium present in theconcentrate preferably varies from a minimum of about 200 grams perliter to the saturation solubility in the concentrate at a temperaturearound 20 C.

In the previously described concentrate it is desirable that not morethan A: of the chromium initially present in the hexavalent state bereduced to the trivalent state.

Thus if sodium sulfite is used as the reducing agent the maximum amountof sodium sulfite in the concentrate previously described should notexceed about 50 grams per liter. The ratio of reducing agent to chromicacid in the concentrate should provide preferably at least 1 gram perliter and not more than 11 grams'per liter of trivalent chromium. l i

The ratio of sulfuric acid to acetic' acid is rather critical and shouldbe approximately 2:1 on a weight basis. In the concentrate the minimumamount of acetic acid is about 20 grams per liter and the maximum amountof acetic acid preferably does not exceed about 40 grams; per liter, theminimum amount of sulfuric acid is about scribed preferably should notbe less than 150 grams per liter nor more than 300 grams per liter- Inthe final metal finishing bath prepared by dilution.

of the concentrate the amountof chromium should be within the range of 3grams to grams per liter, preferably 15 to 90 grams per liter. The ratioof S0; :CrO; should be Within the range of about 1:4 to about 1:16 withoptimum results being obtained at about 55:20. The treating hath made bydiluting the concentrate in the manner previously described Will containabout 0.3 gram per liter to about 0.6 gram per liter of acetic acid andabout 0.6 gram per liter to about 1.2 grams per liter of sulfuric acid.The nitric acid in the treating bath will be about 2.25 grams per literto about 4.5 grams per liter. The weight ratio of sulfuric acid toacetic acid in the metal finishing bath will be the same as the weightratio in the concentrate, namely, approximately 2:1. The trivalentchromium in the treating bath should preferably be at least 0.015 gramper liter and not more than about 4.4 grams per liter.

Instead of the sodium sulfite other reducing agents can .be used whichare capable of reducing hexavalent chro mium compounds to trivalentchromium compounds in' acidic solutions, but it is desirable to employreducing agents which do not form insoluble by-products with the acid ofthe acidic aqueous solution. Other examples of preferred reducing agentsare sodium thiosulfate, sodium hydrosulfite, potassium sulfite,postassium thiosulfate, potassium hydrosulfite, ammonium sulfite,ammonium thiosulfate, and ammonium hydrosulfite. The hydrosulfites arealso commonly referred to as hyposulfites. Other reducing agents whichreduce chromic acid in acid solution can be used. Reducing agents suchas calcium sullite are undesirable because of the formation of insolublecalcium sulfate.

The reduction reaction between the chromate radical of the chromic acidand the sulfite radical which occurs when a sulfite such as sodiumsulfite is mixed with the chromic acid in the presence of water can beexpressed by the following equation:

The invention is especially useful in providing a beautiful bronze colorand enhanced corrosion resistance to Zinc die castings and to other zincsurfaced articles. An important advantage of the invention is that themetal to be treated can be immersed in the treating solution aspreviously described and requires no further treatment other thanrinsing with Water. In many types of chromic acid treatments heretoforeproposed a secondary treatment is required. By the practice of theinvention it is possible to produce zinc surfaced articles which show ahigh resistance to the formation of white corrosion products.

ThGf-BXPI'GSSiOHJ zincsurfaced article is employed herein to cover anyarticle having an exterior surface of'zinoregardlss ofwli'ether thearticle itselfis made of zinc, a ferrous metal or some other metalcapable of being coated-with zinc. "Ihe expression. zinc platedaiti'cleffis employedfhe'rein to describe a zincncoatedr articleinfwhich'the zinc coatinghas' been applid' by, an. electroplatingfprocess;

The invention is. hereby, claimed'as follows llAlpr'ocessofffinishing'zincsurfaced articles wliiclicompri'Sestreatin'gtlie surfaces of such articles with an? aqeuoussolution of chromic :acid containing? 1090" grarn's'pe'r literof"'cliromiiiiriof which at least OLOlS gram per liter but not more thanabout'4i4 grams per literis'chl'oiniuni in a tiiyalent-"state, theremainder being chromiumyin a hexayalnt'st'ate,, Ol'3fto' T6 gram perIiter'of acetic acid} 0161a 1.2"grams per liter of sulfuricacid,lcand.about2l Sgrianis per liter to 4.5 gramsperli'te'r"of'ni't'ric;acid,' the weight ratio of sulfuric acid toacetic acid being approximately 231 and'the weight ratio of S0 :CrO;beingywithin the range of about l:4 to 131'6.

2f A'p'roce's'sas claimed in claim l'in which the articletobe'treatedis'fimmersed in said aqueous solution atIa,

state',.th'e"remainder being chromium in a liexavalent; 35

grams per liter of sulfuric acid and about 2.5 to 4.5

state, 0.3to 016' gram per liter'of acetioacir 016'to 1.2

grams p'er liter of nitric acid," the weight ratio of sulfuricacid" toacetic acidbeing approximately 23 1 and 1' the weight'ratio ofS'OL;":CrOt beingv within the rangeoflz4to 1116.

5i An acidic aqueous 'metal'treating solution consisting essentially ofan aqueous solutionof'chromic acid containing about 15m 90 'grams perliter of chromium ofwhich at least about (LOIS-gram per liter but 'notmore than about 4.4 grams per liter is chromium in a trivalent.

, 4? grams" per" liter of sulfuric acid" and" about 215% 4:5 grams perliter of nitric acid, the weight ratio of sulfuric, acid to acetic acidbeing approximately 2:1 and the weight ratio of SO; !CIO4 being withinthe range of 1:4 to 1:16.

6. A concentrate adapted to be added to water to produce an aqueousmetal? treating solution, said concentrate consisting essentially of anaqueous solution of chromic aeid containing-- chromium in" an? amount- 1from about 200 grams per'litertothesaturatiorr solubility in thesolution -at2v0-? l 0f: which at least 1' grampent liter and not more-:than 1:1- gratn'sper' litristrivalnt chromium, the remainder being,chromium" in. a hertavalent state;'20"to lflvgramsperr'liter of'aceticacid, 41) to grams per liter ofsulfiiridacid and to 300 grams per literof nitricfiacidithe weightiratio of sulfuric acid to acetic acid beingapproximately 2:1 and the weight ratio of S0 ":CrO being within therange of-1 :4to 1:16.

7- A concentrate; as claimed: in claim- 6 in which--= sodium sulfite is:present. in an amount sufiicient; to, pro duce trivalent i chromiumwithin the proportions stated-.2.

8. An acidic aqueous metal treating: concentrate; adap ted tube-dilutedwith. waterto form a metaliprotecting material,= said concentratehavingthe following, approximate compositionr Ingredients? Grams per liter"Chro'm'ic'anhydrideI(CrO)T 401:0 Sodium sulfi'te (Na @80 7:5 Acetic-acid22(5 Sulfuric acid?" t 45:0. Nitric-acid 228.0

the remainder being water.

. References"Gited-in-tlie 'file of this patent UNITEDSTATES-PATENTS2,035,380 Wilhelm' Mar. 24, 1936" 2523,9251 Ostran der' Feb; 17, 19532,796,372 Chester'feflall June 18; T V EOREI-GNPATENTS 586,517Great'Britair'r Mar; 21; 1947" 489,887 Great'Bfitain': July 2, 1947852,634 Germany, Oct, 16,, 19 52:

UNITED STATES PATENT @FFICE CERTIFICATE OF EGRRECTIGN Patent Noa2,902,394 September 1, 1959 Bruno Re Jeremias It is hereby certifiedthat error appears in the-printed specification of the above numberedpatent requiring correction and that the said Letters Patent should readas corrected below.

Column 3, line 35, for "acetic acir" read acetic acid,-- line 4'7, after"state" insert a comma; column 4., line 46, list of references cited,under "FOREIGN PATENTS" for "489,887" read 589,887

Signed and sealed this 23rd day of February 1969 (SEAL) Attest:

KARL Ha AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

1. A PROCESS OF FINISHING ZINC SURFACED ARTICLES WHICH COMPRISESTREATING THE SURFACES OF SUCH ARTICLES WITH AN AQEUOUS SOLUTION OFCHROMIC ACID CONTAINING 3 TO 90 GRAMS PER LITER OF CHROMIUM OF WHICH ATLEAST 0.015 GRAM PER LITER BUT NOT MORE THAN ABOUT 4.4 GRAMS PER LITERIS CHROMIUM IN A TRIVALENT STATE, THE REMAINDER BEING CHROMIUM, IN AHEXAVALENT STATE, 0.3 TO 0.6 GRAM PER LITER OF ACETIC ACID, 0.6 TO 1.2GRMAS PER LITER OF SULFURIC ACID, AND ABOUT 2.25 GRAMS PER LITER OF 4.5GRAMS PER LITER OF NITRIC ACID, THE WEIGHT RATIO OF SULFURIC ACID TOACETIC ACID BEING APPROXIMATELY 2:1 AND THE WEIGHT RATIO OFSO4--:CRO4--BEING WITHIN THE RANGE OF ABOUT 1:4 TO 1:16.